The default unity template isn’t great bout resizing the game’s canvas to fit the shape of the iframe it’s placed in. You can try using this: https://seansleblanc.itch.io/better-minimal-webgl-template

You may also need to make some game code changes to support rendering at a different size, but the template above should help put you in the right direction.

I also notice that you haven’t marked your game as mobile compatible, so keep that in mind if you’re trying to distribute a mobile game, because right now your page will have a warning on phones saying your game isn’t compatible.